Allow extra variables for use with Jinja that are not action parameters.

Sometimes you want to do things like st2 run REQ_FILE="/foo/rqmnts.txt" PIP_OPTIONS="-u" core.local cmd="pip install {{PIP_OPTIONS}} -r {{REQ_FILE}}"

or think @emptywee's use case:

I have an action that will list something, some items I want users to be able to limit the list to show only those that match some string (edited).
I know I can create an action that will do it somehow but with jinja I think I'd have easily made a {% for ..%} loop and showed only those that match the query.

Start by tracing how st2 run handles action parameters and how Jinja expressions are rendered. Clarify how extra variables such as REQ_FILE and PIP_OPTIONS should be supplied, scoped, and validated, then define tests covering the command example and the filtering use case. Done means documented behavior and passing coverage for the agreed interface.